1. Field of the Invention
The present invention is related to the removal, cleaning, and stripping of a wide variety of organic materials that may be deposited or formed on substrates during the manufacture, repair or rework of those substrates, using water-free sulfur trioxide in conjunction with additional processing steps.
2. Description of the Related Art
During the fabrication or repair or rework of various semiconductor and non-semiconductor devices and displays, films or layers of organic materials may be deposited on substrates or wafers that must be removed subsequently. Examples of such devices and displays include, but are not limited to, semiconductor devices and wafers, ceramic devices, liquid crystal display devices, photomasks, flat-panel displays, printed circuit boards, printed wiring boards, magnetic read-write heads, and thin-film read/write heads. Examples of such organic materials include, but are not limited to, photosensitive and non-photosensitive organic materials, polymerized photoresists, cured and uncured polyimides, polycarbonates, paints, resins, multi-layer organic polymers, certain organo-metallic complexes, positive optical photoresists, negative optical photoresists, chemically amplified photoresists, electron-beam photoresists, X-ray photoresists, ion-beam photoresists, and ion-implanted and other hardened photoresists.
The removal, cleaning, or stripping of such organic materials comprising, for example, organic coatings, films, layers, or residues from such substrates or wafers is one of the necessary steps in the manufacture and repair of the devices described above. The removal, cleaning and stripping of such organic materials is usually carried out by one of three general techniques, including (1) dry methods, which include dry-ashing or plasma ashing, dry-stripping, dry-etching, and the use of various procedures which make use of kinetic processes such as abrasives, cryogenic aerosol techniques, CO2 snow, etc.; (2) wet methods, including the so-called RCA clean process (developed by RCA for use in cleaning semiconductor substrates), wet stripping with liquid chemicals such as, for example, sulfuric acid, hydrochloric acid, hydrogen peroxide, piranha etch (a mixture of sulfuric acid and hydrogen peroxide), ozonated deionized water (DI water), and ammonium hydroxide solutions, and the use of organic solvents, for example, various choline solutions, amine-based solutions, M-pyrrole, paint removers, etc.; and (3) a combination of both dry and wet methods, often in repeating sequences.
Dry methods often involve the use of a plasma of high-energy ions to remove organic materials (dry-ashing, or plasma ashing). There are two general categories of plasma methods employed. One of the plasma methods, often referred to as barrel-ashing, makes use of a stream of plasma directed at the substrate. The other method, often referred to as down-stream ashing, involves the use of a plasma gas atmosphere “downstream” (i.e. physically distant from) from the source of the plasma so as to minimize the damage to the substrate. Different plasma gases may be used, including those made up of various mixes of oxygen, ozone, and nitrogen gas, creating CO, CO2 and H2O as end products (see, e.g., Silicon Processing for the VLSI Era, Volume 1—Process Technology, S. Wolf and R. N. Tauber, p 564, Lattice Press, Sunset Beach, Calif., 1986).
In some cases, a hydrogen plasma may be required to assist the dry process. For example, for very difficult-to-remove photoresists, hydrogen plasma may be used to strip the upper layer of hardened resist to create easy-to-strip hydrides (see, e.g. “Choose the Right Process to Strip Your Photoresist”, Semiconductor International, February 1990, p. 83). In other cases, difficult-to-remove residues may require the addition of fluorine gas, or some other halogen gas, to the plasma gas mix, or even a follow-up exposure to hydrofluoric acid vapor (see, e.g. “Managing Etch and Implant Residue”, Semiconductor International, August 1997, p. 62).
Several drawbacks are associated with plasma processes. These include: (1) radiation damage to the underlying substrate, where bombardment of the substrate by high-energy ion plasmas, particularly in the barrel-ashers, can damage the crystal structure of the substrate as well as implant undesired atoms in the substrate, thus reducing yield and reliability of substrate devices (although the damage may be minimized by annealing or by using down-stream ashers which minimize radiation damage at the cost of slower and less effective organic removal rates); (2) creation of additional contamination as high-dose ion plasmas striking impurities in the resist react to form etch-resistant, insoluble inorganic oxides (see, e.g., “New Concerns in Dry Oxygen Ashing”, Semiconductor International, March 1996, p. 44); (3) worsening of the existing contamination typically found in commercial photoresists as the high-energy plasma drives existing metal impurities into the substrate; (4) formation of difficult-to-remove residues such as “via veils”, and “metal fences” and the hardening of sidewall polymers as the result of the interaction of released by-products of plasma etching with the side-walls in the substrate structure at elevated temperatures; and (5) incomplete removal, cleaning and stripping of the photoresists and other organic materials from very small features due to micro-masking of the resists from further processing by sputtered oxides which may form as a result of high-energy ion impact.
Other dry methods are in use which do not require high-energy plasmas. However, these non-plasma methods suffer in general either from (1) low removal rates, (2) high-temperature processing conditions, (3) excessive damage to the substrate, for example, damage from mechanical abrasion caused by micro-sandblasting techniques such as, for example, cryogenic aerosols, the potential for damage created by temperature fluctuations such as, for example, CO2 snow methods (see, e.g. “Emerging Technology; Emerging Markets”, Precision Cleaning, October 1996, p. 14), and damage created by ultraviolet light exposure (UV-exposure), or (4) an inability to completely remove or strip organic materials which have been hardened by exposure to prior processing such as high temperature, or high energy, high dose, ion-implant.
Wet methods, including, for example, the RCA clean, specialized organic solvents, acids, and oxidizing solutions such as Caro's acid, and other liquid reagents, also have a number of drawbacks when used to remove, clean, or strip organic materials. These drawbacks include: (1) incomplete removal of organic materials due to the difficulty all liquids have in penetrating very small features and in overcoming surface tension and capillary cation; (2) incomplete removal due to a limited ability to affect certain organic materials, including photoresists, photoresist residues and organo-metallic complexes which have been hardened by exposure to high energy, high dose ion-implant, or high temperature processing; (3) further introduction of metallic impurities and other residual contamination commonly found in liquid reagents; (4) the spread of contamination to all parts of the substrate, particularly as trace organic residues accumulate in the cleaning solution during the stripping process; (5) the hazardous or toxic nature of many of the organic solvents and acids required; (6) the large volumes of hazardous or toxic reagents which must be maintained in a highly pure condition, often at elevated temperatures; (7) the large number of different types of reagents which must be kept at hand to deal with different cleaning applications and processing conditions; (8) the difficulty and cost of safely disposing of large volumes of hazardous or toxic reagents; and (9) the propensity of many liquid reagents to cause corrosion of the substrate, particularly when metal films are contained in the substrate.
The RCA clean process, a commonly used wet process which involves treatment with NH4OH/H2O2 followed by HCl/H2O2, has similar drawbacks, which limit its effectiveness and application.
Despite the drawbacks of these various methods for removing, cleaning and stripping organic materials, dry methods in combination with wet methods, sometimes requiring several repetitions, must be used, for lack of better methods, to achieve acceptable levels of cleanliness when removing, cleaning and stripping certain very difficult-to-remove organic materials, particularly hardened photoresists. Hardening of photoresists as the result of prior processing is often a problem, making removal, cleaning and stripping difficult. Hardening of photoresists arises from several sources, including (1) exposure to high energy electromagnetic radiation normally used in photolithographic processes and very short wavelength, or deep UV, photoresist curing steps, (2) high energy, high dose ion-implant processes, (3) reactive ion etching processes (RIE), (4) high temperature processes such as postbake, photoresist curing steps, (5) oxide, metal or polysilicon dry etching, as well as other physical and chemical treatments. In addition, dry etching and dry-ashing processes often create extremely etch-resistant polymers and residues of inorganic or organo-metallic materials, such as sidewall polymers, via veils and metal fences (see, e.g., “What's Driving Resist Dry Stripping”, Semiconductor International, November 1994, p. 61). Under such conditions, wet and dry methods in combination may be the only available technique which can provide satisfactory removal, cleaning and stripping of the organic material. Even under conditions where prior art is used in repeated sequential combinations of dry then wet processing, certain organic materials may still be resistant to satisfactory removal. For example, photoresists exposed to oxide etch processes leave carbon-fluorine polymers that are resistant to removal even with successive applications of dry and wet strip processes followed by an RCA clean.
The prior art thus suffers from numerous drawbacks that may be overcome with the teachings of the present invention. Such drawbacks include: (1) difficulty in removing hardened organic materials, including sidewall polymers, via veils, metal fences and other inorganic residues, and photoresists which may have been exposed to high-energy electromagnetic radiation such as UV-hardening (ultraviolet radiation hardening), or high energy, high dose ion-implant, or reactive ion etch (RIE); (2) difficulty in removing organic materials from very small features (generally sub-micron) and high aspect-ratio features without using substrate-damaging plasma methods; (3) the introduction of substrate damage or film erosion when plasma methods must be employed for lack of an effective alternative; (4) the creation of new, removal-resistant inorganic materials when plasma methods must be employed for lack of an effective alternative; (5) the worsening of existing contamination which may be driven into the substrate when plasma methods must be employed for lack of an effective alternative; (6) the introduction of additional contaminants when liquid reagents and solvents are used; (7) the spread of contamination between substrates when liquid reagent and solvent baths are used; (8) the difficulties and expense of buying, using and disposing of large volumes of hazardous or toxic liquid reagents and solvents; (9) the relative complexity of plasma-based methods which require radio-frequency or microwave generators as well as high-vacuum pumps and systems; (10) the difficulty in maintaining a uniform removal process across the diameter of the substrate when barrel-ashers are used and whenever a stripping process must be stopped by the calculation of an optimum end-point; (11) the relatively high temperature of many dry methods (200° C. and up) which can make some diffusion-related problems, such as the diffusion of impurities into the substrate, more severe (both diffusion of impurity materials and consumption of thermal budget may be concerns of the user, depending on the substrate manufacturing process employed); (12) the difficulty of scaling up dry-processes to handle substrates 12 inches in diameter and greater; (13) the difficulty of using the prior art when stripping organic materials from metal films without inviting corrosion of that metal film; (14) the poor selectivity of oxygen plasmas to photoresist over certain organic films lying in close proximity (such as the approximate 1:1 selectivity displayed by oxygen plasmas when used to remove photoresist in close proximity to the interlayer dielectric film material, BCB); and (15) the frequent requirement to develop and operate, complex and expensive multi-step, combination dry plus wet, removal processes in order to adequately clean hardened organic materials from substrates.
U.S. Pat. No. 5,037,506, issued Aug. 6, 1991, to S. Gupta et al and entitled “Method of Stripping Layers of Organic Materials”, discloses and claims a two-step method comprising (1) exposure of organic materials to gaseous sulfur trioxide followed by (2) rinsing with a solvent to remove various organic coatings, polymerized photoresists, and especially implant and deep-UV hardened resist layers, during the manufacture of semiconductor or ceramic devices. While the method disclosed and claimed in this patent is useful, there are further needs for cleaning surfaces and removing organic materials which are not disclosed or claimed in this patent and which extend into other areas of technology. Specifically, those undisclosed and unclaimed needs include the need to remove, clean, and strip organic materials contained on a broad range of substrates including not just semiconductor devices, wafers, ceramic devices and printed circuit boards as suggested in the prior art, but also from substrates used in liquid crystal display devices, photomasks, flat-panel displays, printed wiring boards, magnetic read/write heads, thin-film read/write heads, as well as other substrates upon which organic films may have been deposited and which also contain features (1) where liquid stripping and cleaning methods are inadequate due to surface tension and capillary effects, or due to the contamination introduced and spread by liquids, (2) where plasma techniques result in substrate damage, erosion, or incomplete removal of the organic material, (3) where there is a requirement for improved uniformity of the removal method across the substrate diameter or large dimension than is provided by the prior art, (4) where there is a requirement for the removal of organic materials at a throughput rate which is faster than that provided by prior art, either with an inherently faster organic removal rate, or by providing for very large, batch processing capability, (5) where there is a requirement for more effective removal of silicon polymers, sidewall polymers, via veils, metal fences, and other inorganic residues created by dry etching processes, (6) where there is a requirement for an integrated method for cleaning both organic and inorganic residues with a minimum use of hazardous or toxic and other liquid wastes, (7) where there is a requirement to minimize or eliminate the corrosion of substrate metal films during organic cleaning, (8) where there is a requirement to integrate steps in the stripping and cleaning process in order to improve cycle time, work-in-process, and throughput, (9) where there is a requirement to minimize processing temperatures, or (10) where there is a requirement to remove or strip only part of the organic coating, film or layer, as may be required, as an example, in efforts to planarize or shape that coating, or to remove one organic coating from an underlying organic coating with significant selectivity.
As a result of the passage of time, it has become clear to the present inventors that there are additional considerations regarding the method of the above-mentioned patent that are required in order to improve the effectiveness of the method. As discussed herein, by effectiveness of the method is meant completeness of the organic removal process, elimination of substrate damage and erosion, improved uniformity of processing across the substrate, faster organic removal rate and substrate throughput, an increase in the number of substrates that can be processed simultaneously during one process cycle, minimization of corrosion of the substrate, minimization of total liquid wastes generated by the method, minimization of hazardous or toxic chemical usage, minimization of total process cycle time, minimization of process temperature requirements, and improved selectivity of removal for one type of organic coating lying in close proximity to a second type of organic coating. These additional considerations include: (1) a precursor chemical or physical treatment of the organic material and the substrate may be required prior to insertion of the substrate into the sulfur trioxide reaction chamber for exposure to the process gases; (2) reactive process gases other than inert gases, dry-nitrogen or sulfur trioxide, as specified in prior art, may also be required to be mixed in the sulfur trioxide reaction chamber with the sulfur trioxide; (3) reactive process gases other than sulfur trioxide may also be required to be introduced to the sulfur trioxide reaction chamber in a specific sequence, either before or after introduction of sulfur trioxide; (4) any one of the process gases may also be required to be replenished in the sulfur trioxide reaction chamber at regular intervals during the method; (5) all of the process gases in the sulfur trioxide reaction chamber may also be required to be in movement, and that movement may be required to be in a specific flow pattern, during the method; (6) while in the sulfur trioxide reaction chamber the temperature of the process gases and the substrate may also be required to follow a temperature-time curve; (7) the partial-pressure of any of the process gases in the sulfur trioxide reaction chamber may also be required to follow a partial-pressure versus time curve; (8) simultaneous physical treatment of the substrate (e.g., exposure to high-energy electromagnetic radiation such as ultraviolet light) may also be required during exposure to the process gases while contained within the exposure chamber; (9) it may be necessary to stop the process reactions within the sulfur trioxide reaction chamber prior to their completion and at a precise moment in time; (10) a pre-rinse chemical or physical treatment of the substrate may also be required after exposure to the process gases within the sulfur trioxide reaction chamber but prior to rinsing in a solution to remove reaction products; (11) application of simultaneous physical processes such as, for example, ultrasonic or megasonic processes, or various other kinetic processes, are required while rinsing the substrate in a solution to remove reaction products as described in prior art; and (12) a post-rinse chemical or physical treatment of the substrate may also be required after rinsing in a solution to remove reaction products.
What is needed is a method which satisfies these considerations for effectively removing, stripping, or cleaning organic coatings, films, layers, and residues consisting of, for example, photosensitive and non-photosensitive organic materials, polymerized photoresists, cured and uncured polyimides, polycarbonates, paints, resins, multi-layer organic polymers, certain organo-metallic complexes, positive optical photoresists, negative optical photoresists, chemically amplified photoresists, electron-beam photoresists, X-ray photoresists, ion-beam photoresists, and ion-implanted and other hardened photoresists from a variety of substrates used in devices such as semiconductor devices and wafers, ceramic devices, liquid crystal display devices, photomasks, flat-panel displays, printed circuit boards, printed wiring boards, magnetic read-write heads, and thin-film read/write heads, as well as other substrates upon which organic films may have been deposited.